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KVM: Avoid atomic operations when kicking the running vCPU

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If we do have the vcpu mutex, as is the case if kvm_running_vcpu is set
to the target vcpu of the kick, changes to vcpu->mode do not need atomic
operations; cmpxchg is only needed _outside_ the mutex to ensure that
the IN_GUEST_MODE->EXITING_GUEST_MODE change does not race with the vcpu
thread going OUTSIDE_GUEST_MODE.

Use this to optimize the case of a vCPU sending an interrupt to itself.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Paolo Bonzini 2021-10-20 06:38:05 -04:00
parent fb43496c83
commit aefdc2ed44
1 changed files with 14 additions and 1 deletions
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15
virt/kvm/kvm_main.c
View file

@ -3396,6 +3396,19 @@ void kvm_vcpu_kick(struct kvm_vcpu *vcpu)
if (kvm_vcpu_wake_up(vcpu))
return;
me = get_cpu();
/*
* The only state change done outside the vcpu mutex is IN_GUEST_MODE
* to EXITING_GUEST_MODE. Therefore the moderately expensive "should
* kick" check does not need atomic operations if kvm_vcpu_kick is used
* within the vCPU thread itself.
*/
if (vcpu == __this_cpu_read(kvm_running_vcpu)) {
if (vcpu->mode == IN_GUEST_MODE)
WRITE_ONCE(vcpu->mode, EXITING_GUEST_MODE);
goto out;
}
/*
* Note, the vCPU could get migrated to a different pCPU at any point
* after kvm_arch_vcpu_should_kick(), which could result in sending an
@ -3403,12 +3416,12 @@ void kvm_vcpu_kick(struct kvm_vcpu *vcpu)
* IPI is to force the vCPU to leave IN_GUEST_MODE, and migrating the
* vCPU also requires it to leave IN_GUEST_MODE.
*/
me = get_cpu();
if (kvm_arch_vcpu_should_kick(vcpu)) {
cpu = READ_ONCE(vcpu->cpu);
if (cpu != me && (unsigned)cpu < nr_cpu_ids && cpu_online(cpu))
smp_send_reschedule(cpu);
}
out:
put_cpu();
}
EXPORT_SYMBOL_GPL(kvm_vcpu_kick);